MB-13636: DCP pipeline chokes with in-memory backfills.
MB-13636: DCP pipeline chokes with in-memory backfills.

1. open the connection with DCP.
2. start requesting streams, there is a REQ-LOOP that does
   this for all downstream requested vbuckets.
3. Meanwhile DCP backfill for each stream gets progressively activated
   and start filling the connection.
4. The incoming control & data messages are posted to a buffered channel
   of buffer-size 10,000,
   a. That channel will be read by KVDT routine.
5. Meanwhile on the control path in projector, KVDT gets activated only
   after (2) returns.

With recent DCP overhaul,
a. DcpEvent hops are reduced from 2 to 1 between DCP and projector's
   data-path routine, there by reducing the buffer size from 20,000
   to 10,000 (like mentioned above).
b. And in-memory backfill is populating the connection pretty fast.

The bottom line seem to be that if DCP producer blocks on
connection-write, its connection-read blocks as well there by not
reading out the pending StreamReq.

Change-Id: Id09b3b545dac92474576bb3eb14e4c249d482d5b

jchris/indexing,jchris/indexing,jchris/indexing,jchris/indexing